The invention relates to a catalytic gas phase process for the preparation of epoxides from unsaturated hydrocarbons by oxidation with molecular oxygen in the presence of molecular hydrogen and catalysts for this process which are coated with nanoscale gold particles.
In general, direct oxidations of unsaturated hydrocarbons with molecular oxygen in the gas phase do not proceed below 200xc2x0 C.xe2x80x94even in the presence of catalystsxe2x80x94and it is therefore difficult to prepare oxidation-sensitive oxidation products, such as e.g. epoxides, alcohols or aldehydes, selectively, since the secondary reactions of these products often proceed faster than the oxidation of the olefins employed themselves.
Propene oxide is one of the most important base chemicals of the chemical industry. Its field of use lies in the plastics sector with a proportion of more than 60%, specifically for the preparation of polyether-polyols for the synthesis of polyurethanes. In addition, even greater proportions of the market are covered by propene oxide derivatives in the field of glycols, in particular in lubricants and antifreezes.
About 50% of propene oxide worldwide is currently synthesized via the xe2x80x9cchlorohydrin processxe2x80x9d. A further 50%, with an increasing trend, is supplied by the xe2x80x9coxirane processxe2x80x9d.
In the chlorohydrin process (F. Andreas et al.; Propylenchemie [Propylene chemistry], Berlin 1969), chlorohydrin is first formed by reaction of propene with HOCl (water and chlorine), and propene oxide is then formed from the chlorohydrin by splitting off HCl with a base. The process is cost-intensive, but with appropriate optimization has a high selectivity ( greater than 90%) with high conversions. The loss of chlorine in the chlorohydrin process in the form of worthless solutions of calcium chloride and sodium chloride and the associated high salt load in the waste water led early on to the search for chlorine-free oxidation systems.
The oxidation processes use organic compounds instead of the inorganic oxidizing agent HOCl to transfer oxygen to propene. This indirect epoxidation is based on the fact that in the liquid phase organic peroxides, like hydroperoxides, can transfer their peroxide oxygen selectively to olefins to form epoxides. During this process, the hydroperoxides are converted into alcohols and the peroxycarboxylic acids are converted into acids. Hydroperoxides are produced from the corresponding hydrocarbon by autoxidation with air or molecular oxygen. A serious disadvantage of indirect oxidation is the economic dependence of the propene oxide value on the market value of the coupled product and the cost-intensive preparation of the oxidizing agent.
With titanium silicalite (TS 1) as a catalyst (Notari et al., U.S. Pat. No. 4,410,501 and U.S. Pat. No. 4,701,428), it was possible for the first time to epoxidize propene with hydrogen peroxide in the liquid phase under very mild reaction conditions with selectivities of  greater than 90% (Clerici et al., EP-A 230 949).
Propene oxidation is also achieved with a low yield in the liquid phase on titanium silicalites containing platinum metal with a gas mixture comprising molecular oxygen and molecular hydrogen (JP-A 92/352771).
U.S. Pat. No. 5,623,090 (Haruta et al.) describes a gas phase direct oxidation of propene to propene oxide with a 100% selectivity for the first time. This is a catalytic gas phase oxidation with molecular oxygen in the presence of the reducing agent hydrogen. Commercially available titanium dioxide coated with nanoscale gold particles is used as the catalyst. Nanoscale gold particles here are understood as meaning particles having a diameter in the nm range. The propene conversion and the propene oxide yield are stated as a maximum of 2.3%. The Au/TiO2 catalysts described achieve the approx. 2% propene conversion for only a very short time; e.g. the typical half-lives at moderate temperatures (40-50xc2x0 C.) are still unsatisfactory (Haruta et al., 3rd World Congress on Oxidation Catalysis 1997, p. 965-970, FIG. 6). This process thus has the disadvantage that the yield of epoxide, which is in any case low, is severely reduced further by rapid deactivation.
For economic use, the development of catalysts with significantly better initial activities with a greatly increased catalyst life therefore continues to be absolutely necessary.
The invention therefore provides a process for the oxidation of unsaturated hydrocarbons in the gas phase in the presence of a hydrogen/oxygen mixture, if appropriate with the addition of an inert gas, on a supported catalyst coated with gold particles, characterized in that a calcined catalyst which has been prepared from optionally doped titanium oxide hydrate and is coated with nanoscale gold particles is employed.
The process according to the invention can be used on all olefins. Since the gas phase oxidation expediently takes place at low temperatures ( less than 120xc2x0 C.) on the basis of the higher selectivities which can be achieved, it is possible to oxidize all unsaturated hydrocarbons from which are formed those oxidation products of which the partial pressure is sufficiently low for the product to be removed constantly from the catalyst. Unsaturated hydrocarbons having up to twelve carbon atoms, in particular ethene, propene, 1-butene or 2-butene, are preferred.
The preparation of the catalyst has a decisive influence on the catalyst activity. The catalysts are preferably prepared here by the xe2x80x9cdeposition-precipitationxe2x80x9d method. In this, an aqueous solution of an inorganic or organic gold compound is added dropwise to a stirred aqueous suspension of the titanium oxide hydrate used as the catalyst support. A water-containing solvent is preferably used. Other solvents, such as e.g. alcohols, can also be employed. When bases (e.g. sodium carbonate or alkali metal or alkaline earth metal hydroxide solution) are added to this gold(III) salt solution up to a pH of 7-8.5, gold precipitates out on the titanium oxide hydrate surface in the form of Au(III) chlorohydroxo or oxohydroxo complexes or as gold hydroxide. To bring about a uniform deposition of ultrafine gold particles, the change in the pH must be controlled by slow dropwise addition of this alkaline aqueous solution. Since in an excess of alkali metal hydroxide solution the gold compounds deposited dissolve again to form aurates ([Au(OH)4]31  or AuO2xe2x88x92), for this reason a pH of between 7-8.5 must be established.
Precipitated gold(III) hydroxide cannot be isolated as such, but is converted into the metahydroxide AuO(OH) or Au2O3 on drying, which decomposes to elemental gold with the release of oxygen when calcined above 150xc2x0 C. The nanoscale gold particles generated in this way are immobilized firmly adhering to the support surface, and have particle diameters of  less than 10, preferably  less than 6 nm. The amount of gold applied to the support depends on various variables, thus e.g. on the surface area, on the pore structure and on the chemical nature of the surface of the support. The properties of the support thus play an important role for the catalytic action.
Surprisingly, it has been found that when amorphous hydrated titanium oxide hydrates of high surface area are employed for coating with gold, the catalytic activities in the epoxidation of propene to propene oxide are drastically higher. These titanium oxide hydrates employed have water contents of 5 to 50 wt. % and surface areas of  greater than 50 m2/g. Initial propene oxide yields of  greater than 4% e.g. are obtained with a catalyst which has been prepared on the basis of titanium oxide hydrate and comprises 0.5 wt. % gold.
The water content of the titanium oxide hydrates employed is usually between 5 and 50 wt. %, preferably between 7-20 wt. %. In the preparation of the catalyst, gold is applied to the titanium oxide hydrate in a precipitation step in the form of Au(III) compounds. However, the support loaded in this manner still has no catalytic activity. Only calcining in a stream of air at 350 to 500xc2x0 C. makes a catalytically active material out of this precursor.
Low sulfate contents in the TiO(OH)n precursors surprisingly have the effect of a drastic improvement in the catalytic properties of the catalysts prepared with them. Catalysts based on titanium oxide hydrate with a sulfate content of between 0.1 and 6 wt. %, preferably 0.2-1 wt. %, are therefore preferably employed. The sulfate can be added during the titanium oxide hydrate preparation, or subsequently by treatment of the titanium oxide hydrates with reagents (e.g. sulfuric acid or sodium sulfate).
The concentration of the soluble gold compound in the stirred suspension has a significant influence on the catalytic activity of the catalyst prepared therefrom. By repeating the precipitation operation several times with small amounts of gold (e.g. with 0.5 wt. % gold each time), catalysts with significantly increased catalytic activities compared with catalysts with the same high gold loading which has been applied in one step can be prepared. Catalysts on which amounts of gold of between 0.05 to 10 wt. %, preferably 0.1 to 1 wt. %, have been applied repeatedly to the support by the xe2x80x9cdeposition-precipitationxe2x80x9d process described, after washing and drying, are therefore preferably employed in the process according to the invention. When used in the direct oxidation of propene with molecular oxygen in the presence of molecular hydrogen, the catalyst prepared in this way according to the invention gives propene oxide with yields of  greater than 4% at selectivities of  greater than 97%.
In the preparation of the catalyst, the reduction of the gold hydroxides precipitated on the surface takes place during the calcining. If selected reducing agents (e.g. sodium citrate, magnesium citrate, . . . ) are added during the preparation of the catalyst, the catalytic activities can be increased slightly.
The specific synergistic interaction between the nanoscale gold and the TiO(OH)n support is also achieved if the two components are applied to additional other supports (e.g. SiO2, Al2O3, ZnO).
The amounts of catalyst employed and the amounts of gas employed are not limited. The xe2x80x9cspace velocityxe2x80x9d of the gas stream through the catalyst bed should usually be approx. 0.5 to 20 l/g cat.xc3x97hxe2x88x921.
The process according to the invention is carried out in the presence of the gases oxygen and hydrogen, if appropriate with the addition of inert gas. In the presence of these gases, the oxygenates propene oxide and acetone are also found at 150xc2x0 C., in addition to the main products water, propane and CO2xe2x88x92. At a temperature between 30-60xc2x0 C., only water and traces of other components (approx. 1%, based on PO) are found in addition to the main product propylene oxide (approx. 4% yield).
The composition of the gas phase, comprising propene, oxygen, hydrogen and possibly an inert gas, is important not only for the space/time yield, but also for safety. All molar compositions of the gases propene/oxygen/hydrogen/nitrogen can be employed in theory. Gas mixtures of oxygen and hydrogen are known to be explosive in certain compositions (explosive gas). Surprisingly, it has been found that the oxidation reaction described above can be carried out under approximately xe2x80x9chydrogenating conditionsxe2x80x9d outside the explosion limits. xe2x80x9cHydrogenating conditionsxe2x80x9d means that, in addition to an excess of hydrogen, only very small amounts of oxygen are employed. The following gas ratios are therefore employed for the oxidation reaction: H2/hydrocarbon/oxygen/nitrogen: 20-80 vol. %/10-50 vol. %/1-10 vol. %/0-50 vol. %. Preferably, H2/hydrocarbon/oxygen/nitrogen: 30-75%/15-40%/3-8%/0-10%. The molecular oxygen employed for the reaction can have various origins, e.g. pure oxygen, air or other oxygen/inert gas mixtures.
In addition to the process according to the invention for the oxidation of unsaturated hydrocarbons, the invention furthermore also provides the catalyst employed in this process.